This invention relates to airfoil blades, especially blades for centrifugal blowers, and to methods for their manufacture
The air moving performance of a fan is characterized by the static pressure rise it produces across a range of airflow rates. The curve illustrating the fan pressure rise as a function of flow rate is referred to as the fan characteristic. A typical centrifugal fan characteristic is shown in FIG. 1.
To perform properly, a fan must be matched to its application, or system, at the intended flow-pressure point of operation. The system comprises elements such as ducts, elbows, expanding or converging transitions, heating and cooling coils, screens and guards, dampers, louvers, shutters, nozzles, filters, or bubble pools, all of which produce resistance to airflow. The fan must be designed or specified such that the fan static pressure rise is equal to the sum of the system resistances. The fan operating point is defined as the intersection of the fan and system characteristics, as shown in FIG. 1.
The static pressure rise required for a given application dictates the type of fan to be used. Axial flow fans produce relatively high volume flow rates and relatively low static pressure rise. Centrifugal fans are suited to higher pressure rise applications. The rotating component, or wheel, of a centrifugal fan has a series of blades classified by their curvature and cross section. Of the blade types commonly used today, the airfoil, or AF, blade is the most efficient, and if operated at or near its point of maximum efficiency, has the lowest noise level.
Unfortunately, present airfoil blades are relatively expensive, compared to other types of blades. Backward-curved, backward-inclined, radial-tip, forward-curved, and radial blades all can be produced using simple metal forming operations. The airfoil blade, however, requires additional manufacturing operations, such as bending of a metal plate about a form or pattern. Furthermore, the airfoil blade in most commercial applications is attached to the back plate and ring shroud with a welded bead. These difficult and labor-intensive manufacturing processes often make the cost of the airfoil wheel prohibitive for many customers.
In the contemporary regulatory environment, fan noise and efficiency are receiving increased scrutiny. ASHRAE standards for example, sets limits on the fan power consumption per unit flow rate. Regulations limiting noise are causing fan and HVAC equipment manufacturers to seek improved fan designs, and the increased cooling loads from computers and other electronic equipment have placed enormous demands on air conditioning systems. All of these applications require high efficiency, low noise fans, but contractors and consumers are reluctant to pay the high prices charged for traditional airfoil centrifugal fans.
Japanese Utility Model Publication 62-1437, published on Apr. 13, 1987 to the Japanese Air Conditioned Technical Research Institute for an invention by Eisuke Nishizu, referenced in U.S. Pat. No. 4,971,521, discloses one attempt to solve these problems. Nishizu""s centrifugal blower has extruded airfoil blades, mounted with screws extending through end rings and a back plate into female screw holes in screw fastening portions mounted in ribs extending from the upper skin of the blade to its lower skin.
Unfortunately, the Nishizu design has some shortcomings.
Placing the female screw holes in central ribs means that at least two ribs are required, which means there will be at least three longitudinal cavities through the extrusion. This is contrary to conventional extrusion practice, which seeks to minimize the number of openings in the extrusion. Placing the screw receptors in the cross-ribs also complicates the design and manufacture of the extrusion die(s).
Having all three openings face toward the tail of the blade helps the blade resist forces on the front of the blade, but weakens the mounts with respect to forces that press the blade forward, such as the momentum of the blade if the wheel comes to a sudden stop. Incorporating the screw receptors in the cross ribs moves the screws toward the center of the blade, which increases the torque on the connections from off-center loads.
The invention disclosed herein provides a centrifugal fan wheel with extruded aluminum airfoil blades. The blade extrusion is of the xe2x80x9csemi-hollowxe2x80x9d type, which can be manufactured inexpensively by producing long xe2x80x9cstripsxe2x80x9d of blades that are cut to the desired length. In the preferred embodiment, two hollow regions, separated by an angled mid-load support beam, characterize the inside of the blade. The nose section of the blade (at the leading edge) and the tail section of the blade (at the trailing edge) have open semi-circular grooves in the nose and tail sections, facing the adjacent hollow sections and designed to accept thread cutting or thread forming screws, for attachment of the blade to a ring-shroud and back-plate.
Aluminum extruding is a widely used process, and extruded-aluminum parts are found in many consumer and industrial products. An extrusion die is a relatively low-cost piece of tooling, and holds up well over time, experiencing little wear over a production run of thousands of feet of material. Extruded blade strips can be cut to the desired blade span, allowing wheels of varying widths to be manufactured with little difficulty.
In addition to low-cost tooling and ease of manufacturing, the extruded blade can be designed with internal features, such as bolt holes and structural elements, as described below.
The attachment of the blade with thread cutting or thread forming screws or bolts eliminates the difficulty and cost that is usually associated with this type of wheel. Typical airfoil wheels with large diameters are constructed by welding the blade ends to a front blade support, typically called the ring shroud, and to a rear blade support, typically called the back plate (or center disk, in double-width wheel applications). The semi-hollow extrusion die allows semi-circular holes to be incorporated into the blade interior. These holes are sized in such a way a to facilitate the blade attachment to other components using thread cutting or thread forming screws. In a prototype sample of the invention, aluminum 6061-T6 was used for the blade material, and the required hole diameter tolerance for thread cutting or thread forming screws was maintained. Furthermore, both the drive torque and tightening torque fell within the acceptable range typically specified for this type of screw.
Commercial centrifugal fans are subject to very high structural loads, which sometimes require that supports such as braces, support rings, or other devices be attached to the wheel. Without such support structures, the blade is fixed only at its ends, at the blade-to-ring shroud and blade-to-back-plate attachment points. At the blade mid-chord, mid-span location, farthest from the attachment points, the centrifugal forces give rise to large stress and deflection of the blade.
The extruded-blade construction allows a variety of internal features to be integrated into the airfoil-blade design. Using finite-element analysis (FEA) of the structural loads on the blade, we decided to add a mid-chord structural support beam to the airfoil cross section, connecting the pressure and suction sides of the hollow blade. The effectiveness of the beam is increased by angling it slightly, such that it is substantially aligned with the radial force vector imposed by the centrifugal load. In practice, the designer would set the pitch angle of the blade for the desired air moving performance, and align the beam with a straight line connecting the wheel center of rotation with the mid-chord location of the airfoil.
In summary, a low-cost, high-efficiency, and low-noise centrifugal fan airfoil (AF) blade has been invented. The blade is suitable for use in both single- and double-width centrifugal wheels. The unique design, featuring extruded-aluminum construction, an integral structural support beam, and semi-circular holes for blade attachment with thread cutting or thread forming screws, represents a simple, low-cost alternative to traditional AF centrifugal fan blades.
Other features and advantages of this invention will be apparent from the following detailed description.